A transformer has become an essential electronic component for various kinds of electric appliance. Referring to FIG. 1, a schematic exploded view of a conventional transformer is illustrated. The transformer 1 principally comprises a magnetic core assembly 11, a bobbin 12, a primary winding coil 13 and a secondary winding coil 14. The primary winding coil 13 and the secondary winding coil 14 are wounded around the bobbin 12. A tape 15 is provided for isolation and insulation. The magnetic core assembly 11 is generally shaped as an EE-type core, an EI-type core or an ER-type core. The middle portions 111 of the core 11 are embedded into the cylinder tube 121 of the bobbin 12. The primary winding coil 13 and the secondary winding coil 14 interact with the magnetic core assembly 11 to achieve the purpose of voltage regulation.
Since the leakage inductance of the transformer has an influence on the electric conversion efficiency of a power converter, it is very important to control leakage inductance. Related technologies were developed to increase coupling coefficient and reduce leakage inductance of the transformer so as to reduce power loss upon voltage regulation. In the transformer of FIG. 1, the primary winding coil 13 and the secondary winding coil 14 are superimposed with each other and wounded around the bobbin 12. As a consequence, there is less magnetic flux leakage generated from the primary winding coil 13 and the secondary winding coil 14. Under this circumstance, since the coupling coefficient is increased, the leakage inductance of the transformer is reduced and the power loss upon voltage regulation is reduced, the electric conversion efficiency of a power converter is enhanced.
In the power supply system of the electric products for the new generation, for example LCD televisions, the transformer with leakage inductance prevails. The current generated from the power supply system will pass through a LC resonant circuit composed of an inductor L and a capacitor C. The inductor L is provided from the primary winding coil of the transformer. Meanwhile, the current with a near half-sine waveform will pass through a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch. When the current is zero, the power MOSFET switch is conducted. After a half-sine wave is past and the current returns zero, the switch is shut off. As known, this soft switch of the resonant circuit may reduce damage possibility of the switch and minimize the noise.
In order to increase the leakage inductance of the transformer, the primary winding coil should be separated from the secondary winding coil by a certain distance to reduce the coupling coefficient of the transformer. Referring to FIG. 2, a schematic exploded view of a transformer with leakage inductance according to prior art is illustrated. The transformer 2 principally comprises a bobbin 21, a primary winding coil 22 and a secondary winding coil 23. The bobbin 21 comprises a first side plate 211, a second side plate 212 and a winding member 213. A tape 24 is wound around the middle portion of the winding member 213 and has a width d. The winding member 213 is divided into a first winding portion 2131 and a second winding portion 2132, which are located at bilateral sides of the tape 24. The primary winding coil 22 and the secondary winding coil 23 are wound around the first winding portion 2131 and the second winding portion 2132, respectively. The first winding portion 2131 is separated from the first side plate 211 by wrapping a first side tape 25 on the winding member 213 between the first winding portion 2131 and the first side plate 211. Likewise, the second winding portion 2132 is separated from the second side plate 212 by wrapping a second side tape 26 on the winding member 213 between the second winding portion 2132 and the second side plate 212. For safety regulations, the tape 24 is used for isolation between the primary winding coil 22 and the secondary winding coil 23. Via the first side tape 25 and the second side tape 26, the primary winding coil 22 and the secondary winding coil 23 are electrically isolated from the conductors outside the transformer 2. As the width d of the tape 24 between the primary winding coil 22 and the secondary winding coil 23 is increased, the coupling coefficient is reduced and the leakage inductance of the transformer is increased. Under this circumstance, the resonant circuit of the power supply system will be conveniently controlled.
Although the transformer structure of FIG. 2 is advantageous for increasing the leakage inductance, some drawbacks still exist. As previously described, the magnitude of the leakage inductance is dependent on the width d of the tape 24 between the primary winding coil 22 and the secondary winding coil 23. Since the tape 24 is made of flexible material and fails to be firmly fixed, the structure of the transformer is readily distorted due to a long-term using period or serious vibration. Under this circumstance, the magnitude of the leakage inductance is reduced or unstable, and the resonant circuit of the power supply system will be adversely affected. Since these tapes are sticky and narrow in width, the procedures of wrapping the tape 24, the first side tape 25 and the second side tape 26 are labor-intensive and complicated. In addition, if the wrapping result is unsatisfied, the electrical performance of the transformer is impaired.
Since the tape 24, the first side tape 25 and the second side tape 26 are wrapped on the winding member 213 of the bobbin 21, the remaining area or volume for winding the primary winding coil 22 and the secondary winding coil 23 around the winding member 213 is limited and thus the heat-dissipating effect is usually insufficient. Furthermore, after the procedures of winding the coils and wrapping the tapes, a layer of insulating tape is additionally wrapped around the primary winding coil 22 and the secondary winding coil 23. The insulating tape also impairs heat dissipation of the transformer during operation. Moreover, since the melting point of the tape 24 is relatively lower, the operating temperature of the transformer is restricted by the melting point of the tape 24.
With increasing development of electronic technologies, the electric conversion efficiency of a power converter to be used in an electronic product is gradually demanding. For example, in a case that a voltage is intended to be converted from a low voltage (e.g. 400V) to a high voltage (e.g. 2,000V), for meeting the requirement of safety regulations, the distance between the primary winding coil and the secondary winding coil should be increased to avoid conduction between the primary winding coil and the secondary winding coil. Unfortunately, since the width d of the tape 24 is insufficient and the converted voltage is too high, the conduction between the primary winding coil and the secondary winding coil is possible.
In views of the above-described disadvantages, the applicant keeps on carving unflaggingly to develop a structure of a transformer according to the present invention through wholehearted experience and research.